Typically, vent valve devices are employed in modern day freight car brake systems because of the ever increasing length of brake pipe resulting from longer cars, such as "piggy-back" cars, articulated cars, and other inter-modal specialty cars. While freight car control valve devices also include an integral vent valve device, the fact that one or more cars in succession along the train may have their brake systems cut out of operation makes the emergency reduction wave of brake pipe pressure through the entire train difficult to sustain without auxiliary means of venting the brake pipe pressure.
It will be understood, of course, that failure of a brake pipe pressure reduction wave to be propagated through the train at a sufficiently fast rate will preclude attainment of an emergency brake application. It is important, therefore, that a vent valve device be positive in its operation to assure that propagation of an emergency brake pipe pressure reduction wave is sustained through the entire length of the train.
It is also important that, in assuring positive operation in response to an emergency rate of reduction of brake pipe pressure, a vent valve device must have sufficient stability as to not respond to service rates of brake pipe pressure reduction.
Finally, the vent valve device should be insensitive to different train "running" pressures, i.e., the normal pressure at which the train brake pipe is charged. Trains can be operated at pressures of 70 psi to 110 psi.
Vent valve devices, such as shown and disclosed in U.S. Pat. No. 3,165,115, generally operate by sensing a pressure differential across a piston valve assembly that only arises when the rate of reduction of brake pipe pressure on one side of the piston assembly is at an emergency rate.
Two principles of operation are well known for preventing such a pressure differential from arising when a service rate of brake pipe pressure reduction is in effect. In one case, a reference pressure on the other side of the piston assembly is vented directly to atmosphere and in the other case the reference pressure is vented via the evacuated brake pipe. In each case a choke limits the rate of venting of this reference pressure to a service rate, so that only when the brake pipe pressure is reduced at an emergency rate is a predetermined pressure differential developed across the piston assembly. When this occurs, the piston assembly is actuated to operate a valve device that exhausts brake pipe pressure locally. The principle of operation discussed in the first case, i.e., venting the reference pressure directly to atmosphere, requires, for optimum performance, the use of a special valve to throttle the exhaust of the reference pressure during service braking in accord with the actual rate of brake pipe pressure reduction in effect.
When employing the principle of venting the reference pressure via the brake pipe, as noted in the other case mentioned above, the brake pipe pressure influences the rate via which the reference pressure is vented, thus making operation of a vent valve device according to this principle of operation intrinsically sensitive to the different pressures carried in the train brake pipe. While this characteristic is disadvantageous, a vent valve device having this principle of operation does not require a special valve to obtain maximum efficiency and thus can be considerably less costly to build and maintain.
Moreover, U.S. Pat. No. 4,043,604 shows and discloses a vent valve/emergency valve device in which a differential area piston assembly is employed to compensate for different brake pipe pressures, as a means of overcoming the pressure sensitivity of vent valve devices that operate on the principle of venting a reference pressure via the brake pipe.